Acids donate H⁺ in aqueous solution. The number of moles of H⁺ donated per mole of acid is the proticity of the acid. For example, sulfuric acid is diprotic as it donates two moles of H⁺ per mole of acid. In this experiment you will carry out a titration to determine the proticity of citric acid, C6H8O7. FB 1 is aqueous citric acid containing 7.50 g dm⁻³ C6H8O7. FB 2 is aqueous sodium hydroxide containing 4.50 g dm⁻³ NaOH. FB 3 is thymolphthalein indicator.

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

1. [1] I The following data are recorded • two burette readings AND titre for the rough titration • initial and final burette readings for two (or more) accurate titrations
2. [1] II Titre values recorded for accurate titrations, AND Correct headings and units in the accurate titration table • initial / start AND (burette) reading / volume • final / end AND (burette) reading / volume • titre OR volume used / added / OR FB 2 used / added • unit: / cm³ OR (cm³) OR in cm³ (for each heading) OR cm³ unit given for each volume recorded.
3. [1] III All accurate burette readings recorded to 0.05 cm³
4. [1] IV The final accurate titre recorded must be within 0.10 cm³ of any other accurate titre
5. [3] Accuracy (Q) marks Round burette readings to the nearest 0.05 cm³. Check and correct titre subtractions where necessary. Select the best mean titre, using the following hierarchy: • two (or more) accurate identical titres (ignoring any that are labelled 'rough'), then • two (or more) accurate titres within 0.05 cm³, then • two (or more) accurate titres within 0.10 cm³, etc Calculate the Supervisor's mean titre to two decimal places. Calculate the candidate's mean titre to two decimal places. Calculate the difference (δ) between the supervisor's and candidate's mean titre. Award accuracy Q marks as follows: V Award if δ < 0.50 cm³ VI Award if δ < 0.30 cm³ VII Award if δ < 0.20 cm³
6. [1] Correctly calculates mean titre • Candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm³. • Working/explanation must be shown OR ticks must be put next to the two (or more) accurate readings selected. • The mean should be quoted to 2 dp and be rounded to nearest 0.01 cm³. (e.g. 26.675 cm³ must be rounded to 26.68 cm³)
7. [2] Correctly uses (answer to 3 or 4 sf) M1: Amount citric acid in titration = 25/1000 × 7.50 / 192 M2: 9.766 × 10⁻⁴ mol OR 9.77 × 10⁻⁴ mol
8. [1] Correctly calculates Amount of NaOH used = (b) / 1000 × 4.5 / 40 mol AND answer to 3 or 4 sig fig
9. [1] Calculated mole ratio = (c)(ii) / (c)(i) OR stated mole ratio of 2.5 ≤ mole ratio ≤ 3.5 AND results support the statement / acid is triprotic. OR stated mole ratio < 2.5 or ≥ 3.5 AND results do not support the statement / acid is not triprotic.
10. [1] Equation C6H8O7 + 3NaOH → C6H5O7Na3 + 3H2O
11. [1] Any valid displayed/structural/skeletal formula containing 3 COOH groups Expected answer is HOOCCH2C(OH)COOHCH2COOH
12. [1] Student is not correct AND uncertainty in burette volume = 2 × 0.05 (which is greater than 0.06)